MONDAY, March 25 (HealthDay News) -- Older adults who harbor certain infections, such as the herpes cold sore virus, may have poorer thinking and memory abilities than their peers, a new study suggests.

Researchers found that of more than 1,600 older adults, those with signs of chronic infection with herpes simplex and certain other viruses and bacteria scored lower on standard tests of mental skills.

But the findings, published in the March 26 issue of Neurology, do not prove the infections are to blame.

"They could just be bystanders," said lead researcher Dr. Mira Katan, a neurologist with Columbia University College of Physicians and Surgeons in New York City.

"We can't make any definite conclusions about causality," Katan said. "At this stage, you can just say there's an association."

Still, it's an "interesting" association that needs further study, she said.

Many people carry herpes simplex virus, or HSV. One form, HSV-1, usually causes cold sores around the mouth, while HSV-2 is the main cause of genital herpes. Once a person is infected with either form of HSV, the virus remains dormant in the body's nerve cells and can be reactivated repeatedly.

HSV can also move to any part of the body, including the brain, and for several decades, some researchers have speculated that chronic HSV infection might contribute to dementia -- possibly by causing persistent inflammation.

In the new study, Katan's team used blood samples from 1,625 older adults -- average age 69 -- to look for indicators of chronic infection with a few common pathogens: HSV and another virus in the herpes family called cytomegalovirus, which usually causes no symptoms; C. pneumoniae, a bacterium that causes respiratory infections; and H. pylori, a stomach-dwelling bacterium that can cause ulcers.

On average, the greater their "infection burden," the worse the older adults performed on a standard test of thinking and memory, the study found.

That was true even when the researchers weighed other factors that affect older adults' mental sharpness, including education, smoking, heart disease and diabetes.

But the study hinted that exercise might play a protective role. The research team found that infection "burden" was related to mental impairment only among sedentary people -- and not those who said they got some exercise.

However, that too needs to be studied further, the team noted.

Katan said that infection with the viruses, rather than the two bacteria, seemed to play a greater role in mental decline. Overall, 23 percent of the study participants had signs of mental impairment at the study's start; the odds of impairment were 2.5 times higher among people who carried all three viruses -- HSV 1 and 2, and cytomegalovirus -- than for people who carried only one virus.

"It's not just infection with one virus," Katan said. "It's the overall burden."

Infections alone would not be the whole story, either, said Dr. Timo Strandberg, a researcher at the University of Helsinki in Finland, whose own work has uncovered a link between herpes viruses and dementia.

Most people carry at least one of these pathogens; up to 80 percent of U.S. adults are infected with cytomegalovirus by age 40, for example. But not all develop dementia.

"Cognitive [mental] decline in old age is multifactorial," said Strandberg, who co-wrote an accompanying journal editorial.

For example, the gene variant APO E4 makes people more vulnerable to developing Alzheimer's disease, and one theory has been that if infections can promote mental decline, APO E4 carriers would be more susceptible.

There was no evidence of that in this study, however.

Strandberg agreed that more research is needed to know whether herpes viruses or other infections really contribute to mental decline.

In this study infections were linked to older adults' mental test scores at the study's start -- but not to changes in their test performance over the next eight years. It's not clear why that was, Katan's team noted.

Strandberg said he thinks only a controlled clinical trial can give answers. He said a first step could be a trial where Alzheimer's patients are randomly assigned to take an antiviral drug to prevent reactivation of herpes viruses.

But Katan said any intervention would presumably have to happen earlier. "If they already have dementia, it's too late. The damage would be done," she said.

If further studies do confirm that certain viruses play a role in mental decline, prevention would be important. Katan pointed to vaccines as a potential way to protect people, "but we're a long way off from that," she said.

The study was funded by U.S. and Swiss government grants. Some researchers on the work and Strandberg, the editorialist, have ties to several drug companies.

SOURCES: Mira Katan, M.D., neurologist, Columbia University College of Physicians and Surgeons, New York City; Timo Strandberg, M.D., Ph.D., University of Helsinki and University Hospital, Finland; March 26, 2013, Neurology